What you have to remember is that because you can do it, doesnt mean that it is a good idea. The problem with cutting the ceiling joists is that they are part of a "triangle" of the frame that help to support the tremendous weight of the roof. When you cut any wood that supports weight such as the ceiling joists or floor joists (if you have a wooden foundation) you are weakening the entire support structure. A lot also depends on how exactly the weight of the roof is distributed. You might get lucky and cut joists where the weight is less than on other joists, but it is NOT a good idea to try it. Think of it this way, you paid a lot of money for your house and it is better to think of an alternative for placing attic stairs than have to pay major repair costs later.